1. Field of the Invention
The present invention relates generally to electrical fuses and particularly to surface mount chip fuses employing a new and improved multi-layer, multi-element monolithic construction. The invention farther relates to the methods for manufacturing and fabricating such fuses.
2. Description of the Related Art
The utilization of surface mount components on printed circuit boards has become the preferred method for circuit board assembly as device geometries continually decrease in size and overall circuit density continues to increase. Several advantages of surface mount components over older leaded devices (used in through-hole technology) include decreased size, decreased weight and lower profile. Additionally, the use of surface mount components generally lowers manufacturing costs by allowing the use of highly automated assembly equipment. The shift from leaded components to surface mount components by the electronics industry has resulted in greater demands for smaller, higher reliability, less costly surface mount fuses with greater amperage and voltage ratings.
Surface mount fuses allow the circuit designer to protect critical (and oftentimes expensive) components and circuitry at the board level or subsystem level rather than relying on an external fuse or some other external current protection device. Overload current may result from a wide variety of sources. Short circuit conditions can occur in filter capacitors, supply lines or output loads which cause overload current flow. To provide adequate protection to sensitive electronics, the clear-time characteristics of surface mount fuses used in these applications must be very fast and extremely predictable.
Conventional surface mount chip fuses have numerous limitations. The most notable limitation is amperage rating. Generally, surface mount chip fuses (e.g., 1206 size and smaller) are limited to ratings of 5.0 amperes and less. 1206 refers to a component size which is approximately 120 mils in length by 60 mils in width. This terminology is common in the chip component industry (others include 0805, 0603, and 0402). One reason for the amperage limitations of traditional chip fuses is related to the fusing element employed by these devices. Many of these devices utilize thin film techniques for deposition of the fusing element.
A typical example of this type of fuse is disclosed in U.S. Pat. No. 5,296,833 to Breen et al. This patent discloses a thin film fuse which is constructed by depositing a thin metal layer on an insulative substrate, bonding an insulative coating to the upper surface of the insulative substrate and bonding secondary cover layers of greater mechanical strength to the bottom and top of entire assembly. The object of the Breen et al. device is to provide a thin film fuse of higher voltage rating than previously possible while maintaining quick clear-times. A second object of the Breen et al. invention is to provide a thin film fuse structure that has improved mechanical strength and reliability, as well as greater thermal cycling ability.
Other conventional surface mount chip fuses have been fabricated with wire fusing elements. Amperage ratings for these devices are controlled by increasing or decreasing the diameter of the fusible wire. A typical example of this type of fuse is disclosed in U.S. Pat. No. 5,440,802 to Whitney et al. This patent discloses a method of manufacturing chip fuses in which a plurality of spaced, parallel columns of electrically-conductive film are deposited on an upper surface of a green ceramic plate. A plurality of electrically-conductive wire elements are deposited on the upper surface of the plate in a mutually parallel spaced relationship and substantially perpendicular to the film columns. A cover plate of unfired ceramic material is bonded to the upper surface of the plate. The cover plate covers the film columns and wire elements, to form a laminate structure. The laminate structure is divided, to form a plurality of individual fuses. The fuses are fired to cure the ceramic, and to create an inter-metallic bond between the wire elements and the conductive metal film at mutual points of contact.
Fuses constructed in this manner are limited to lower amperage and voltage ratings (less than 5.0 amps and 36 VDC) due to their limited ability to suppress arcs which may occur during the clearing action when an overload current is present. The reason for these amperage/voltage limitations is that during a clearing action (e.g., the "blowing" of the fuse), the fuse element material should be completely absorbed by the surrounding substrate layers. If the fuse element material protrudes from the surrounding substrate layers, the fuse material may continue to allow large amounts of current to pass and not effectively open the circuit.
Higher amperage devices require larger diameter fusing wires to handle steady state load currents. Thus, more fuse element material must be displaced when a 5.0 amp fuse is cleared than when a 1.0 amp fuse is cleared. The larger mass of element material utilized in the higher amperage devices often results in an increased probability that the surrounding arc suppressive layer will become saturated with the fusing material during a clearing action which can lead to catastrophic failure (e.g., a fuse which does not properly open a given circuit).
Some conventional non-surface mount fuses and fuse assemblies have addressed the need for higher voltage and amperage capabilities through the use of multiple fusing elements. For example, U.S. Pat. No. 5,479,147 to Montgomery (one of the present inventors) discloses a fuse assembly comprising a plurality of thick film elements or a combination of thick film and wire fusing elements which are formed on a single substrate in an electrically parallel configuration to provide for higher voltage/current capability. Although the fuses described in U.S. Pat. No. 5,479,147 provide higher voltage capability, the package size must be larger to accommodate the wider fusing elements.